In recent years, a salt having a melting point at near roan temperature or a salt having a melting point, less than room temperature (ionic liquid) is found. The ionic liquid is comprised of a cation and an anion, and exists as a liquid state even at room temperature because a bonding strength is very weak. When the structures of a cation and an anion are designed as a bonding strength become poor, it is possible to vary a melting point of the salt and obtain an ionic liquid. Furthermore, it is said that the properties of an ionic liquid can be controlled intentionally by varying a combination of a cation and an anion, or introducing a substituent into each ion.
Ionic liquids are hard to volatilize and have the characteristic that they exist stably to high temperatures more than several hundred degrees centigrade. Ionic liquids differ from so-called “liquid” such, as water or organic solvent in characteristics and are called “third liquid”. Use of the ionic liquids as a lubricant or application to a reaction solvent or extraction separation medium are investigated employing hard volatilization and excellent thermal stability of the liquids. In addition, the ionic liquid is salt and has ionic conductivity because it is comprised only of ions. Therefore, it is possible to use ionic liquid itself as an electrolytic solution. Investigation is actively conducted for using an ionic liquid as an electrolytic solution of a battery or a capacitor, or as a plating bath. Conventionally an aqueous electrolytic solution or an organic electrolytic solution has been used for an electrolytic solution of a battery and a capacitor, the aqueous electrolytic solution will foe restricted in water decomposition voltage, and the organic electrolytic solution encounters to a problem in heat resistance and safety. Ionic liquid are preferable in view of safety because they are incombustible and nonvolatile, and are also high in electrochemical stability, they are suitable as an electrolytic solution of an electric double layer capacitor or a battery to use under particularly high temperature environment.
Ionic liquids composed of various cations and anions are investigated in order to apply ion liquids as an electrolytic solution of a battery and a capacitor. Recently, the characteristic of an ionic liquid which is 1-ethyl-3-methylimidazolium difluorophosphate having difluorophosphate as an anion was reported (Non-patent Literature 1). It is disclosed that 1-ethyl-3-methylimidazolium difluorophosphate is equal in electrical conductivity and voltage resistance to 1-ethyl-3-methylimidazolium tetrafluoroborate which is known as a representative ionic liquid, and is reported that it is suitable as an electrolytic solution of an electric double layer capacitor (Non-patent Literature 2).
In Non-patent Literature 1, 1-ethyl-3-methylimidazolium chloride is reacted, with potassium difluorophosphate in acetone, potassium chloride which is a by-product is removed by filtration, the resulting acetone solution is passed through an alumina column, and acetone is distilled off to obtain 1-ethyl-3-methylimidazolium difluorophosphate. Performance of a battery or a capacitor is remarkably influenced by impurities in an electrolytic solution, and thus it is desirable to reduce impurities as low as possible when an ionic liquid is used as an electrolytic solution. Ionic liquids are hardly volatile and are liquid state in a wide temperature range. It is difficult to reduce impurities in the ionic liquid by purification such, as distillation or recrystallization. Therefore, it is necessary to use a starting material having high purity in order to prepare an ionic liquid having high purity. Potassium difluorophosphate used in Non-patent literature 1 is desirable to contain impurities as low as possible.
Processes for preparing difluorophosphoric acid salt are disclosed, for example, in Patent Literatures 1 to 5 and Non-patent Literatures 3 to 7.
In Patent Literature 1, a process is disclosed for preparing potassium difluorophosphate by mixing and melting potassium hexafluorophosphate and potassium metaphosphate. However, this process is not deemed an excellent process in view of product purity and productivity because of contamination of impurities from a melting pot and high-temperature environment of 700° C.
Patent Literatures 2 to 5 disclose processes for preparing lithium difluorophosphate by reacting lithium hexafluorophosphate or phosphorus pentafluoride with any of lithium metaphosphate, silicon dioxide or lithium carbonate in an organic electrolytic solution. However, these reactions require 40 to 170 hours for obtaining difluorophosphoric acid salt and are not suitable for industrial production.
Non-patent Literature 3 or 4 discloses a process for preparing difluorophosphoric acid salt by reacting diphosphorus pentaoxide with ammonium fluoride or acid sodium fluoride. However, these processes produce a lot of monofluophosphate, phosphate and water as by-products, require severe purification process and are not, effective methods. Non-patent Literature 5 discloses a process for preparing difluorophosphoric acid salt by reacting P2O3F4 (difluorophosphoric acid anhydride) with oxide or hydroxide such as Li2O or LiOH. However, difluorophosphoric acid anhydride used herein is very expensive and high-purity one is hardly available, and thus this process is unfavorable for industrial production.
Non-patent Literature 6 discloses a process for preparing difluorophosphoric acid salt by reacting alkali metal chloride with excess of difluorophosphoric acid and removing hydrogen chloride (by-product) and unreached difluorophosphoric acid by drying with heat at a reduced pressure. However, it is difficult to obtain difluorophosphoric acid salt having high purity even if starting difluorophosphoric acid having high purity is used, since a lot of monofluorophosphoric acid salt or fluoride salt remains as impurities in the desired difluorophosphoric acid salt.
Non-patent Literature 7 discloses a process for preparing potassium difluorophosphate by melting and reacting potassium dihydrogenphosphate and ammonium fluoride. The reaction temperature is about 170° C. and is mild compared with the reaction condition of patent Literature 1, hence easy to practice industrially. However, it is not effective in view of treatment of a large quantity of by-produced ammonia gas and remaining of a large quantity of ammonium fluoride. Thus, there is problem in the purity of the final product.
Further, difluorophosphoric acid salt having high purity is usable not only as a starting material for an ionic liquid but as an additive for an electrolytic solution of a lithium secondary battery. In recent years, in the application field of the lithium secondary battery, a further technological advance is seen in the improvement of output density and the energy density, and the restriction of capacity loss with use expansion of the electronic equipment such as a mobile phone, personal computer, digital camera to one in-vehicle use. Particularly, since the products in-vehicle use might be exposed to the environment that is snore severe than those of consumer products use, high reliability is required in a life cycle and storage characteristics of the products. A non-aqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent, is used as an electrolytic solution fox a lithium secondary battery. Because the non-aqueous electrolytic solution decomposes and causes extraordinary reaction to influence on the performance of the lithium secondary battery, it is attempted to improve life cycle and storage characteristics of the battery by adding various additives to the non-aqueous electrolytic solution. Patent Literature 6 discloses it is possible to form a film on a positive electrode and a negative electrode by using a non-aqueous electrolytic solution containing as an additive at least one of lithium monofluorophosphate and lithium difluorophosphate and to suppress decomposition of the electrolytic solution caused, by the contact of the non-aqueous electrolytic solution and a positive electrode active material and negative electrode active material. As a result, it is possible to suppress the self discharge and enhance storage characteristics.